U.S. patent application number 11/278637 was filed with the patent office on 2007-03-29 for method of operating laser light source.
This patent application is currently assigned to Advanced Mask Inspection Technology Inc.. Invention is credited to Kazuto Matsuki.
Application Number | 20070071042 11/278637 |
Document ID | / |
Family ID | 37893877 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070071042 |
Kind Code |
A1 |
Matsuki; Kazuto |
March 29, 2007 |
METHOD OF OPERATING LASER LIGHT SOURCE
Abstract
A method of operating a laser light source including a
wavelength conversion device in which two wavelength laser beams
are input to a nonlinear crystal to output a sum frequency
wavelength, according to an embodiment of the present invention,
comprises: inputting only one wavelength laser beam of the two
input wavelength laser beams to the nonlinear crystal; measuring
scattered light intensity of the one wavelength laser beam by a
light sensitive sensor installed on an optical axis of the sum
frequency wavelength output beam; and judging a damage state of the
nonlinear crystal based on a measurement value obtained by the
intensity measurement by the light sensitive sensor.
Inventors: |
Matsuki; Kazuto; (Tokyo,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Advanced Mask Inspection Technology
Inc.
Kawasaki-shi
JP
|
Family ID: |
37893877 |
Appl. No.: |
11/278637 |
Filed: |
April 4, 2006 |
Current U.S.
Class: |
372/22 ;
372/38.01 |
Current CPC
Class: |
G02F 1/3525
20130101 |
Class at
Publication: |
372/022 ;
372/038.01 |
International
Class: |
H01S 3/10 20060101
H01S003/10; H01S 3/00 20060101 H01S003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2005 |
JP |
2005-281108 |
Claims
1. A method of operating a laser light source including a
wavelength conversion device in which two wavelength laser beams
are input to a nonlinear crystal to output a sum frequency
wavelength, the method comprising: intermitting stationary use of
the laser light source with any interval; inputting only one
wavelength laser beam of the two wavelength laser beams to the
nonlinear crystal; measuring scattered light intensity of the one
wavelength laser beam by a light sensitive sensor installed on an
optical axis of the sum frequency wavelength output beam; and
judging a damage state of the nonlinear crystal based on a
measurement value obtained by the measuring.
2. A method of operating a laser light source according to claim 1,
wherein the one wavelength laser beam has a shorter wavelength than
the other.
3. A method of operating a laser light source including a
wavelength conversion device in which two wavelength laser beams
are input to a nonlinear crystal to output a sum frequency
wavelength, the method comprising: intermitting stationary use of
the laser light source with any interval; inputting only one
wavelength laser beam of the two wavelength laser beams to the
nonlinear crystal; measuring scattered light intensity of the one
wavelength laser beam by a light sensitive sensor installed on an
optical axis of the sum frequency wavelength output beam; inputting
only the other wavelength laser beam different from the one
wavelength laser beam to the nonlinear crystal; measuring scattered
light intensity of the other wavelength laser beam by the light
sensitive sensor installed on the optical axis of the sum frequency
wavelength output beam; and judging a damage state of the nonlinear
crystal based on both a measurement result of the scattered light
intensity of the one wavelength laser beam and a measurement result
of the scattered light intensity of the other wavelength laser
beam.
4. A method of operating a laser light source including a
wavelength conversion device having a nonlinear crystal which is a
wavelength conversion element and a heater which maintains the
nonlinear crystal at a constant temperature, the method comprising:
measuring electric power supplied to the heater; and judging a
damage state of the nonlinear crystal based on a measurement value
obtained by the measurement.
5. A method of operating a laser light source according to claim 4,
wherein the judgment of the damage state of the nonlinear crystal
is performed based on deviance from correlation of power of laser
input beam and electric power supplied to the heater in the
measurement value of the electric power supplied to the heater.
6. A method of operating a laser light source according to claim 4,
wherein the judgment of the damage state of the nonlinear crystal
is performed based on deviance from correlation of power of laser
input beam, a set temperature, and electric power supplied to the
heater in the measurement value of the electric power supplied to
the heater.
7. A method of operating a laser light source according to claim 4,
wherein the wavelength conversion device is a wavelength conversion
device in which two wavelength laser beams are input to the
nonlinear crystal to output a sum frequency wavelength.
8. A method of operating a laser light source according to claim 4,
wherein the wavelength conversion device is a wavelength conversion
device in which one wavelength laser beam is input to the nonlinear
crystal to output a harmonic wavelength.
9. A method of operating a laser light source including a
wavelength conversion device having a nonlinear crystal which is a
wavelength conversion element and a heater which maintains the
nonlinear crystal at a constant temperature, the method comprising:
measuring electric power supplied to the heater; judging a damage
state of the nonlinear crystal based on a measurement value
obtained by the measurement; and automatically moving the nonlinear
crystal so that a laser beam to be input to the nonlinear crystal
is received at a non-damaged region of the nonlinear crystal.
10. A method of operating a laser light source according to claim
9, wherein the judgment of the damage state of the nonlinear
crystal is performed based on deviance from correlation of power of
laser input beam and electric power supplied to the heater in the
measurement value of the electric power supplied to the heater.
11. A method of operating a laser light source according to claim
9, wherein the judgment of the damage state of the nonlinear
crystal is performed based on deviance from correlation of power of
laser input beam, a set temperature, and electric power supplied to
the heater in the measurement value of the electric power supplied
to the heater.
12. A method of operating a laser light source according to claim
9, wherein the wavelength conversion device is a wavelength
conversion device in which two wavelength laser beams are input to
the nonlinear crystal to output a sum frequency wavelength.
13. A method of operating a laser light source according to claim
9, wherein the wavelength conversion device is a wavelength
conversion device in which one wavelength laser beam is input to
the nonlinear crystal to output a harmonic wavelength.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based on and claims priority of Japanese
Patent Application (JPA) No.2005-281108, filed on Sep. 28, 2005,
the entire contents of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method of operating a
laser light source for use in various kinds of inspections,
measurement, and process and, more particularly, to a method of
operating a laser light source in a deep ultraviolet region using a
nonlinear crystal.
[0004] 2. Description of the Background
[0005] Generally, a laser beam has characteristics that since the
frequency is higher than that of an electric wave, information
storage capacity is large; furthermore, since the wavelength is
equal and the phase is uniform, a laser beam is excellent in
monochromaticity and directivity and has coherency which is not
seen for a normal beam; further, since a laser beam can be
extremely thinly converged, energy is focused on a minute area to
realize high temperature and high voltage locally and
instantaneously, resulting in application to many fields such as
communications and information fields, measurement field,
application to process technology, and application to medical
field.
[0006] In the laser for use in various usages, more specifically,
in a usage where a short wavelength is required, a laser in a deep
ultraviolet region is used. However, since there are limitations of
a laser medium and an excitation source in the deep ultraviolet
region, laser oscillation is difficult. Consequently, as for a
laser light source in the deep ultraviolet region, there are many
methods which perform wavelength conversion of laser output of
visible region from relatively stable near-infrared. That is, it is
a method which uses harmonic generation and sum frequency
generation using a nonlinear crystal.
[0007] In the case of the harmonic generation, laser output of
integral multiple frequency such as frequencies 2.omega. and
3.omega., that is, laser output of a wavelength which becomes short
to .lamda./2 and .lamda./3, with respect to an input wavelength
.lamda., can be obtained by inputting a laser beam of a frequency
.omega. to a nonlinear optical crystal.
[0008] Furthermore, in the case of the sum frequency generation,
laser output whose frequency is .omega.3 (=.omega.1+.omega.2), that
is, laser output of .lamda.3 whose wavelength is shorter than that
of either input wavelength .lamda.1 or .lamda.2 that fulfills a
relationship, 1/.lamda.3=1/80 1+1/.lamda.2, with respect to input
wavelengths .lamda.1 and .lamda.2, can be obtained by inputting a
laser beam of two different frequencies .omega.1 and .omega.2 to
the nonlinear crystal.
[0009] Naturally, BBO crystal (BaB.sub.2O.sub.4) and CLBO crystal
(CsLiB.sub.6O.sub.10), which are nonlinear crystal for use in sum
frequency generation and harmonic generation in a deep ultraviolet
region, are subject to damages due to light or circumference
moisture. Then, if damages are generated on the nonlinear crystal,
it causes degradation of laser beam output and change in intensity
distribution. So, there arises a problem in that measurement
accuracy and process quality deteriorate in the inspection,
measurement, and process with laser beam.
[0010] Consequently, in the case where a nonlinear crystal is
generally operated for a long time, a crystal with a certain degree
of size is used; and, when damage is generated, the nonlinear
crystal is moved so that a non-damaged part is used. Therefore, the
nonlinear crystal is installed on a micromotion mechanism.
Furthermore, the nonlinear crystal is installed in a space so as to
purge circumferential atmosphere and is heated so as to be a
constant temperature with a heater or the like for preventing it
from being damaged due to moisture.
[0011] As described above, in the method in which the nonlinear
crystal is moved each time the nonlinear crystal is damaged, it is
indispensable to judge whether or not damage is generated.
[0012] For example, an operation method, in which a constant time
is set based on the evaluation result separately carried out and
the crystal is moved when an operating time of a laser light source
exceeds the predetermined time, is adopted. However, since this
method generally sets a sufficient margin for damage, the crystal
is actually moved regardless of no degradation of laser output;
therefore, there is a problem in that operating life of an
expensive nonlinear crystal cannot be effectively used.
Furthermore, as a result, there arises a problem in that effective
use of a laser device cannot be made because maintenance frequency
of the laser device increases.
[0013] Consequently, in a laser device using a nonlinear crystal, a
method, in which a part of laser beam irradiating a workpiece is
separated and the separated laser beam is observed to monitor
damage of the nonlinear crystal, is disclosed in a document (for
example, refer to Japanese Patent Application, Publication No.
2003-46173). FIG. 8 is a configuration view showing a laser device
of a prior art. In FIG. 8, a laser beam is output from a laser
oscillator 82 by electric power supplied by a laser power supply
89. Then, the laser beam output from the laser oscillator is split
by a beam splitter 88. Then, intensity of one laser beam of the
split laser beams is measured by a light sensitive sensor 84.
Degradation of the laser beam intensity, that is, the presence or
absence of damage of the nonlinear crystal is judged by monitoring
change in this laser beam intensity.
[0014] For a certainty, according to this prior art, damage of the
nonlinear crystal for use in the laser oscillator can be monitored
by monitoring change in the laser beam output simultaneously with
laser processing. However, a scattered light which is generated
when the nonlinear crystal is damaged is certainly superimposed on
the laser beam output. Therefore, there is a problem in that if
degradation of the laser beam due to damage of the nonlinear
crystal is equivalent to the order of increase in scattered light,
both are offset, so that it becomes truly difficult to judge the
presence or absence of damage of the nonlinear crystal.
BRIEF SUMMARY OF THE INVENTION
[0015] The present invention has been made in view of the
circumstances and an object of the present invention is to provide
a method of operating a laser light source capable of accurately
monitoring damage of a nonlinear crystal for use in a laser light
source and stably providing a laser beam.
[0016] A method of operating a laser light source including a
wavelength conversion device in which two wavelength laser beams
are input to a nonlinear crystal to output a sum frequency
wavelength, according to an embodiment of the present invention,
comprises:
[0017] intermitting stationary use of the laser light source with
any interval;
[0018] inputting only one wavelength laser beam of the two
wavelength laser beams to the nonlinear crystal;
[0019] measuring scattered light intensity of the one wavelength
laser beam by a light sensitive sensor installed on an optical axis
of the sum frequency wavelength output beam; and
[0020] judging a damage state of the nonlinear crystal based on a
measurement value obtained by the measuring.
[0021] A method of operating a laser light source including a
wavelength conversion device in which two wavelength laser beams
are input to a nonlinear crystal to output a sum frequency
wavelength, according to an embodiment of the present invention,
comprises:
[0022] intermitting stationary use of the laser light source with
any interval;
[0023] inputting only one wavelength laser beam of the two
wavelength laser beams to the nonlinear crystal;
[0024] measuring scattered light intensity of the one wavelength
laser beam by a light sensitive sensor installed on an optical axis
of the sum frequency wavelength output beam;
[0025] inputting only the other wavelength laser beam different
from the one wavelength laser beam to the nonlinear crystal;
[0026] measuring scattered light intensity of the other wavelength
laser beam by the light sensitive sensor installed on the optical
axis of the sum frequency wavelength output beam; and
[0027] judging a damage state of the nonlinear crystal based on
both a measurement result of the scattered light intensity of the
one wavelength laser beam and a measurement result of the scattered
light intensity of the other wavelength laser beam.
[0028] A method of operating a laser light source including a
wavelength conversion device having a nonlinear crystal which is a
wavelength conversion element and a heater which maintains the
nonlinear crystal at a constant temperature, according to an
embodiment of the present invention, comprises:
[0029] measuring electric power supplied to the heater; and
[0030] judging a damage state of the nonlinear crystal based on a
measurement value obtained by the measurement.
[0031] A method of operating a laser light source including a
wavelength conversion device having a nonlinear crystal which is a
wavelength conversion element and a heater which maintains the
nonlinear crystal at a constant temperature, according to an
embodiment of the present invention, comprises:
[0032] measuring electric power supplied to the heater;
[0033] judging a damage state of the nonlinear crystal based on a
measurement value obtained by the measurement; and
[0034] automatically moving the nonlinear crystal so that a laser
beam to be input to the nonlinear crystal is received at a
non-damaged region of the nonlinear crystal.
[0035] According to the present invention, it becomes possible to
provide a method of operating a laser light source capable of
accurately monitoring damage of a nonlinear crystal for use in a
laser light source and stably providing a laser beam.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to a
first embodiment and a laser device provided with the light
source;
[0037] FIG. 2 is a view for explaining the operation method
according to the first embodiment;
[0038] FIG. 3 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to a
second embodiment and a laser device provided with the light
source;
[0039] FIG. 4 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to a
third embodiment and a laser device provided with the light
source;
[0040] FIG. 5 is a view showing a relationship between input light
and electric power supplied to a heater;
[0041] FIG. 6 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to a
variant of the third embodiment and a laser device provided with
the light source;
[0042] FIG. 7 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to a
fourth embodiment and a laser device provided with the light
source; and
[0043] FIG. 8 is a conceptual view showing a configuration of a
prior art.
DETAILED DESCRIPTION OF THE INVENTION
[0044] A method of operating a laser device according to the
present invention will be described below based on the accompanying
drawings.
[First Embodiment]
[0045] First, as a first embodiment according to the present
invention, there will be described a method of operating a laser
light source including a wavelength conversion device in which two
wavelength laser beams are input to a nonlinear crystal to output a
sum frequency wavelength, the method including: a step of
intermitting stationary use of the laser light source with any
interval; a step of inputting only one wavelength laser beam of the
two wavelength laser beams to the nonlinear crystal; a step of
measuring scattered light intensity of the one wavelength laser
beam by a light sensitive sensor installed on an optical axis of
the sum frequency wavelength output beam; and a step of judging a
damage state of the nonlinear crystal based on a measurement value
obtained by the measurement of the scattered light intensity.
(Device Configuration)
[0046] FIG. 1 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to the
first embodiment and a laser device provided with the light
source.
[0047] A laser device 100 according to the present embodiment
includes a laser light source 101. The laser light source 101
includes a first laser oscillator 2 which generates a .lamda.1
wavelength laser beam, a second laser oscillator 3 which generates
a .lamda.2 wavelength laser beam, a first shutter 9 which blocks
the .lamda.1 wavelength laser beam output from the first laser
oscillator 2, a second shutter 10 which blocks the .lamda.2
wavelength laser beam output from the second laser oscillator 3, a
nonlinear crystal 1 which is a wavelength conversion element for
generating a .lamda.3 wavelength laser beam by sum frequency
mixture of the input wavelengths .lamda.1 and .lamda.2, and a
micromotion device 7 which moves the crystal when damage is
generated on the nonlinear crystal 1.
[0048] Furthermore, the laser device 100 according to the present
embodiment includes a light sensitive sensor 4 which is disposed on
an optical axis of the .lamda.3 laser beam output from the laser
light source 101, a judging circuit 5 which judges a damage state
of the nonlinear crystal based on a measurement value obtained by
the light sensitive sensor 4, a sample to be irradiated 6 to which
the .lamda.3 laser beam output from the laser light source 101 is
irradiated, and a movable mirror 8 by which an optical path is
changed when the laser beam is irradiated to the sample to be
irradiated.
[0049] In FIG. 1, those other than component parts necessary for
explaining the first embodiment are not shown. It goes without
saying that the laser device 100 and the laser light source 101
generally include other necessary components.
[0050] Here, for example, the first laser oscillator 2 uses a YAG
(Yttrium Aluminum Garnet) laser oscillator in which neodymium
serves as an active medium, oscillating at wavelength 1064 nm; and,
the second laser oscillator 3 uses an argon laser oscillator which
generates a harmonic two times of 244 nm. Then, CLBO crystal
(CsLiB.sub.6O.sub.10) is used as the nonlinear crystal 1 of the
wavelength conversion element. Consequently, a laser beam output of
a wavelength 198.5 nm (=1/(1/1064+1/244)) can be obtained from the
laser light source 101 as a sum frequency. If such a laser beam in
a deep ultraviolet region is used, for example, when the laser
device 100 is a defect inspecting device in which the sample to be
irradiated 6 is used as a mask pattern for use in a semiconductor
lithography, sufficient resolution characteristic can be obtained
even in a mask of 65 nm design rule product which is a
next-generation semiconductor product, thereby enabling it as an
inspection object.
[0051] Here, the first laser oscillator 2 and second laser
oscillator 3 are not particularly limited to a YAG laser or argon
laser oscillator, but a ruby laser, titanium sapphire laser, or
other laser oscillator may be used provided that it is capable of
generating a sum frequency laser beam by sum frequency mixture.
Furthermore, the nonlinear crystal 1 is not limited to CLBO
crystal, but BBO crystal (BaB.sub.2O.sub.4), LBO crystal
(LiB.sub.3O.sub.5), or other crystal may be used provided that it
is crystal capable of generating a sum frequency laser beam. Then,
the light sensitive sensor 4 may be such that laser beam intensity
can be monitored. For example, it may be a power meter which
measures total energy received at a light receiving surface in the
case where a photodiode is assumed as a light receiving element; or
it may be a beam profiler in which a plurality of CCD sensors
two-dimensionally regularly arranged are assumed as light receiving
elements. Further, the laser device 100 is not also limited to a
mask defect inspecting device if it is a device such that a laser
beam is irradiated to the sample to be irradiated 6; for example, a
semiconductor exposure device (stepper), optical disk device,
length measurement device using laser interference, laser
processing device, or other device may be used. Furthermore, here,
a configuration which changes an optical path of the laser beam is
used by providing the movable mirror 8 when the laser beam is
irradiated to the sample to be irradiated; but, it is not
necessarily limited to this configuration. For example, as a prior
art, the laser beam may be separated for irradiating the sample and
the light sensitive sensor 4 by using a fixed half mirror.
Alternatively, it may be configured such that a movable type light
sensitive sensor is inserted to an optical path of the laser beam
only when the laser beam is monitored.
(Operation Method)
[0052] FIG. 2 is a view for explaining a method of operating a
laser light source.
[0053] Stationary use (routine use of inspection, measurement,
process, and the like by laser output beam irradiation to the
sample to be irradiated 6) of the laser light source is intermitted
with any interval in order to monitor a damage state of the
nonlinear crystal 1. For example, intermittence for monitoring may
be performed for every use, every several times to some dozen
times, or every predetermined accumulated time of use of a laser
light source.
[0054] When a damage state of the nonlinear crystal 1 is monitored,
of two wavelength laser beams of the wavelenmgth .lamda.1 and
.lamda.2 to be input, here, one laser beam with the wavelenmgth
.lamda.1 is blocked by the first shutter 9. In this case, even when
the laser beam of wavelength .lamda.2 is input to the nonlinear
crystal 1, laser output beam of wavelength .lamda.3 of sum
frequency is not generated; therefore, intensity of the beam to be
input to the light sensitive sensor 4 provided on the optical axis
of the laser output beam of .lamda.3 is generally zero. Naturally,
the nonlinear crystal 1 is subject to damages due to light
input/output during use of the laser light source or circumference
moisture. Then, since damage of crystal is strain of a crystal
lattice or noncrystallization of the crystal, scattered light is
generated from the damaged part to its periphery when a laser beam
of wavelength .lamda.2 is input to the damaged part. Therefore,
this scattered light is also input to the light sensitive sensor 4.
The scattered light intensity has a positive correlation with
damage degree of the nonlinear crystal 1. Consequently, it becomes
possible to judge movement time of the nonlinear crystal 1 by the
judging circuit 5 based on a measurement value of the light
sensitive sensor 4 by previously determining the scattered light
intensity which shows limitation of damage degree of the crystal as
a threshold.
[0055] Then, when the judging circuit 5 judges that it is a moving
time of the nonlinear crystal 1, the position of the nonlinear
crystal 1 is moved so that the two wavelength laser beams to be
input are received at a non-damaged region of the crystal using the
micromotion device 7. Then, in the case where a non-damaged region
does not exist any more on the nonlinear crystal 1, the nonlinear
crystal 1 is replaced.
[0056] Here, in the case where monitoring of the scattered light
intensity is performed by only one wavelength laser beam, it is
desirable to be performed by a shorter wavelength laser beam of two
wavelengths because scattering is readily generated.
[0057] Furthermore, it is desirable that not only one laser beam
but both wavelength scattered lights are alternatively monitored by
the light sensitive sensor 4 and the movement time of the crystal
is judged based on both measurement results, because information
volume is increased and judgment accuracy is enhanced.
(Effects)
[0058] According to the present embodiment, damage judgment of the
nonlinear crystal, whose accuracy has not been enhanced due to a
scattered light in the known monitoring of the laser output beam,
can be performed with higher accuracy by an operation method by
which judgment is performed based on only scattered light
intensity. Then, the laser beam can be supplied more stably
compared to the prior art by moving the nonlinear crystal based on
damaged judgment with high accuracy. Furthermore, since the light
sensitive sensor which monitors intensity of the laser beam output
from the laser light source is generally provided with a laser
device, an additional light sensitive sensor need not be provided
when the operation method according to the present embodiment is
applied; therefore device configuration is not complicated.
[Second Embodiment]
[0059] Next, as a second embodiment according to the present
invention, there will be described a method of operating a laser
light source including a wavelength conversion device in which two
wavelength laser beams are input to a nonlinear crystal to output a
sum frequency wavelength, the method including: a step of
automatically intermitting stationary use of the laser light source
with any interval and inputting only one wavelength laser beam of
the two wavelength laser beams to be input, to the nonlinear
crystal; a step of measuring scattered light intensity of the one
wavelength laser beam by a light sensitive sensor installed on an
optical axis of the sum frequency wavelength output beam; a step of
judging a damage state of the nonlinear crystal based on a
measurement value obtained by the measurement of the scattered
light; and a step of automatically moving the nonlinear crystal so
that a laser beam to be input is received at a non-damaged region
of the nonlinear crystal.
(Device Configuration)
[0060] FIG. 3 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to the
second embodiment and a laser device provided with the light
source.
[0061] The difference from the first embodiment is in that there
are included a micromotion device control circuit 11 which receives
an input from the judging circuit 5 and automatically controls the
micromotion device 7 and a shutter blocking circuit 17 which
controls the first shutter 9 and the second shutter 10.
(Operation Method)
[0062] Stationary use (routine use of inspection, measurement,
process, and the like by laser output beam irradiation to the
sample to be irradiated 6) of the laser light source is intermitted
with any interval in order to monitor a damage state of the
nonlinear crystal 1. This intermittence is automatically performed
with predetermined interval or timing. Then, when stationary use is
intermitted for monitoring, the first shutter 9 is blocked by the
shutter blocking circuit 17. After that, it is the same as in the
first embodiment in that the movement time of the nonlinear crystal
1 is judged by the judging circuit 5 based on a measurement value
from the light sensitive sensor 4. Here, in the case that it is
judged as the movement time of the nonlinear crystal 1 by the
judging circuit 5, its information is automatically input to the
micromotion device control circuit 11. Then, the position of the
nonlinear crystal 1 is automatically moved by the micromotion
device 7 so that the laser input beam is received at a non-damaged
region based on a signal output from the micromotion device control
circuit 11. At this time, it is more preferable if an operation
method is such that alarm of crystal replacement is automatically
generated provided that the non-damaged region does not already
remain on the nonlinear crystal 1.
(Effects)
[0063] In addition to the effects of the first embodiment, the
stationary use is automatically intermitted and, in the case that
it is judged as the movement time of the nonlinear crystal 1, the
crystal is automatically moved; therefore, time loss is reduced
compared to the case where an operator manually moves and it is
possible to increase an operating rate.
[Third Embodiment]
[0064] Next, as a third embodiment according to the present
invention, there will be described a method of operating a laser
light source including a wavelength conversion device having a
nonlinear crystal which is a wavelength conversion element and a
heater which maintains the nonlinear crystal at a constant
temperature, the method including: a step of measuring electric
power supplied to the heater; and a step of judging a damage state
of the nonlinear crystal based on a measurement value obtained by
the measurement of the supplied electric power.
(Device Configuration)
[0065] FIG. 4 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to the
third embodiment and a laser device provided with the light
source.
[0066] A laser device 100 according to the present embodiment
includes a laser light source 101. The laser light source 101
includes a first laser oscillator 2 which generates a .lamda.1
wavelength laser beam, a second laser oscillator 3 which generates
a .lamda.2 wavelength laser beam, a nonlinear crystal 1 which is a
wavelength conversion element for generating a .lamda.3 wavelength
laser beam by sum frequency mixture of the wavelengths .lamda.1 and
.lamda.2, and a micromotion device 7 which moves the crystal when
damage is generated on the nonlinear crystal 1. Further, in order
to prevent the nonlinear crystal 1 from being damaged due to
absorption of circumference moisture and to suppress fluctuation
due to temperature, which is a characteristic of laser beam, a
heater 13 and a temperature sensor 12 are provided for maintaining
the nonlinear crystal 1 at a constant temperature.
[0067] Furthermore, the laser device 100 according to the present
embodiment, as a temperature maintenance mechanism of the nonlinear
crystal 1, includes a temperature control circuit 14 which controls
temperature of the heater 13 and a power supply 15 which is an
electric power supply source to the heater 13 based on information
from the temperature sensor 12. Furthermore, there are included a
wattmeter 16 which monitors electric power supplied to a heater 13,
a judging circuit 5 which judges a damage state of the nonlinear
crystal based on a measurement value obtained from the wattmeter
16, a sample to be irradiated 6 to which the .lamda.3 laser beam
output from the laser light source 101 is irradiated, and a mirror
8 which changes an optical path when the laser beam is irradiated
to the sample to be irradiated 6.
[0068] In FIG. 4, those other than component parts necessary for
explaining the third embodiment are not shown. It goes without
saying that the laser device 100 and the laser light source 101
generally include other necessary components.
[0069] Here, for example, if the first laser oscillator 2 uses a
YAG laser oscillator in which neodymium serves as an active medium,
oscillating at wavelength 1064 nm; and, the second laser oscillator
3 uses an argon laser oscillator which generates a harmonic two
times of 244 nm; a laser beam output of a wavelength 198.5 nm can
be obtained from the laser light source 101. Then, it is the same
as in the first embodiment in that if such a laser beam in a deep
ultraviolet region is used, it becomes possible to inspect a mask
of 65 nm design rule product which is a next-generation
semiconductor product, for example.
[0070] Here, the first laser oscillator 2 and second laser
oscillator 3 are not particularly limited to a YAG laser or argon
laser oscillator, but a ruby laser, titanium sapphire laser, or
other laser may be used provided that it is capable of generating a
sum frequency laser beam by sum frequency mixture. Furthermore, the
nonlinear crystal 1 is not limited to CLBO crystal, but BBO crystal
(BaB.sub.2O.sub.4), LBO crystal (LiB.sub.3O.sub.5), or other
crystal may be used provided that it is crystal capable of
generating a sum frequency laser beam. Then, although the electric
power supplied to the heater 13 is monitored by the wattmeter 16,
it is not necessarily limited to a wattmeter; but a measurement
device such as an amperemeter may be used provided that electric
energy supplied from the power supply 15 to the heater 13 can be
monitored. Further, the laser device 100 is not also limited to a
mask defect inspecting device as long as it is a device such that a
laser beam is irradiated to the sample to be irradiated 6; for
example, a length measurement device using laser interference,
laser processing device, or other device may be used. Furthermore,
here, a configuration which changes an optical path of the laser
beam is used by providing the movable type mirror 8 when the laser
beam is irradiated to the sample to be irradiated, but, it is not
necessarily limited to this configuration. For example, the laser
beam may be directly irradiated to the sample to be irradiated
without bending the optical path or the optical path may be changed
by a plurality of mirrors.
(Operation Method)
[0071] Generally, the nonlinear crystal 1 has a constant negative
correlation between electric power supplied to the heater 15 and
input light power because a part of the input light is converted to
heat. FIG. 5 is a view showing a frame format of a relationship
between input light power and electric power supplied to the heater
in the case where one power of two wavelength laser beams to be
input to the nonlinear crystal 1 is fixed and the other power is
changed to control power of the laser output beam. Closed circles
in the drawing show the relationship between the both in the case
where damage is not generated in the nonlinear crystal 1. As is
apparent from the drawing, when the power of the input light is
increased, the supply electric energy to the heater necessary for
constantly maintaining temperature of the nonlinear crystal 1 is
decreased. Naturally, if damage is generated on the nonlinear
crystal 1 due to a long term use of the laser light source 101,
light-heat conversion is generated more frequently than usual at
the damaged part. Consequently, the electric power supplied to the
heater 13 for maintaining the nonlinear crystal 1 at a constant
temperature decreases; and, it deviates from the correlation in the
case where damage is not generated on the nonlinear crystal 1 as
shown by a open circle in FIG. 5.
[0072] In the operation method according to the present embodiment,
calibration of the electric power supplied to the heater 13
necessary for maintaining the nonlinear crystal 1 at a constant
temperature with respect to the power of the laser input beam to be
used is performed in advance. Then, from the calibration, a
threshold of the supplied electric power which is judged that the
nonlinear crystal 1 is damaged to some degree to be moved, that is,
a value which is judged that it significantly deviates from the
correlation of the power of the input light and the electric power
supplied to the heater is determined and input to the judging
circuit 5 as a judgment threshold.
[0073] Then, electric power to be supplied from the power supply 15
to the heater during using the laser light source 101 is monitored
by the wattmeter 16 and the measured value is compared with the
threshold at the judging circuit 5, so that it is possible to judge
a movement time of the nonlinear crystal 1 at the judging circuit
5. Then, if it is judged that it is the movement time of the
nonlinear crystal 1 at the judging circuit 5, the nonlinear crystal
1 is moved using the micromotion device 7 to a position at which
two wavelength laser beams to be input are received at a
non-damaged region of the nonlinear crystal 1. Then, in the case
where a non-damaged region does not exist any more on the nonlinear
crystal 1, the nonlinear crystal 1 is replaced.
[0074] In addition, the judgment of the movement time of the
nonlinear crystal 1 is not necessarily limited to a method based on
deviance of the electric power supplied from correlation of one
variable in which only power of the input light with a set
temperature fixed is as a valuable; for example, in the case where
a set temperature is tuned by a light receiving position of the
crystal, a method based on deviance of the electric power supplied
from correlation of two variables of power of the input light and a
set temperature may be applied.
(Effects)
[0075] According to the present embodiment, damage judgment of the
nonlinear crystal, whose accuracy has not been enhanced due to a
scattered light in the known monitoring of the laser output beam,
can be performed with higher accuracy by an operation method by
which judgment is performed based on fluctuation of electric power
supplied to a heater. Then, the laser beam can be supplied more
stably compared to the prior art by moving the nonlinear crystal
based on damaged judgment with high accuracy. Furthermore, since a
damage state of the nonlinear crystal 1 can be monitored in a
stationary use state, it is not necessary to particularly provide a
time for monitoring and therefore efficient use of the device can
be made. Further, at the application of the operation method of the
present embodiment, since a temperature maintenance mechanism
generally provided with a laser device is diverted, changes in new
device configuration such as addition of a wattmeter and change in
a judging circuit configuration are limited and therefore the
device configuration is not complicated.
[Variant of Third Embodiment]
[0076] Next, as a variant of a third embodiment, there will be
described a case where an operation method which performs judgment
of damage of a nonlinear crystal based on fluctuation of electric
power supplied to a heater is applied to a laser light source which
generates a harmonic.
(Device Configuration)
[0077] FIG. 6 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to a
variant of the third embodiment and a laser device provided with
the light source.
[0078] Configuration other than those in which the laser light
source does not use a laser light source which generates a sum
frequency but use a laser light source which generates a harmonic,
is the same as in the third embodiment.
[0079] A laser light source 101 includes a first laser oscillator 2
which generates a .lamda.1 wavelength and a nonlinear crystal 1
which is a wavelength conversion element for generating a .lamda.2
wavelength laser beam.
[0080] Here, for example, the laser oscillator 2 uses a YAG laser
oscillator in which neodymium serves as an active medium,
oscillating at wavelength 1064 nm. Then, CLBO crystal
(CsLiB.sub.6O.sub.10) is used as the nonlinear crystal 1 of the
wavelength conversion element. Consequently, a laser beam output of
a wavelength 532 nm (=1/(1/1064+1/1064)) can be obtained from the
laser light source 101 as a second harmonic of a laser beam of a
wavelength 1064 nm.
[0081] In addition, the laser oscillator 2 is not particularly
limited to a YAG laser oscillator, but a ruby laser, titanium
sapphire laser, or other laser may be used provided that it is
capable of generating a harmonic laser beam by entering to the
nonlinear crystal. Furthermore, a mode, in which a fundamental wave
output from the laser oscillator 2 is converted to a harmonic and
the converted one is input to the nonlinear crystal 1 to be further
converted to a short wavelength harmonic, may be used. Then, the
nonlinear crystal 1 is not also limited to CLBO crystal, but BBO
crystal (BaB.sub.2O.sub.4), LBO crystal (LiB.sub.3O.sub.5), or
other crystal may be used provided that it is crystal capable of
generating a harmonic laser beam.
(Operation Method)
[0082] An operation method of the variant according to the present
embodiment is the same as in the third embodiment except that a
laser beam to be entered to the nonlinear crystal 1 during
calibration is only one wavelength.
(Effects)
[0083] According to the variant of the present embodiment, damage
judgment of the nonlinear crystal, which is used for harmonic
generation whose accuracy has not been enhanced due to a scattered
light in the known monitoring of the laser output beam, can be
performed with higher accuracy by an operation method by which
judgment is performed based on fluctuation of electric power
supplied to a heater. Then, the laser beam can be supplied more
stably compared to the prior art by moving the nonlinear crystal
based on damaged judgment with high accuracy. In addition, it is
the same as in the third embodiment in that effective use of the
laser device can be made and the device configuration is not
complicated.
[Fourth Embodiment]
[0084] Next, as a fourth embodiment according to the present
invention, there will be described a method of operating a laser
light source including a wavelength conversion device having a
nonlinear crystal which is a wavelength conversion element and a
heat-retention mechanism which maintains the nonlinear crystal at a
constant temperature, the method including: a step of measuring
input parameters to a heater of the heat-retention mechanism; a
step of judging a damage state of the nonlinear crystal based on a
measurement value obtained by the measurement; and a step of
automatically moving the nonlinear crystal so that a laser beam to
be input is received at a non-damaged region of the nonlinear
crystal.
(Device Configuration)
[0085] FIG. 7 is a conceptual view showing a configuration of a
laser light source operated by an operation method according to the
fourth embodiment and a laser device provided with the light
source.
[0086] It differs from the third embodiment in that an input is
received from the judging circuit 5 and the micromotion device
control circuit 11 automatically controls the micromotion device
7.
[0087] In addition, although the laser light source 101 is a sum
frequency in FIG. 7, the sum frequency is not necessarily used, but
a harmonic may be used.
(Operation Method)
[0088] It is the same as in the first embodiment in that the
movement time of the nonlinear crystal 1 is judged by the judging
circuit 5 based on a measurement value from the wattmeter 16. Here,
in the case that it is judged as the movement time of the crystal
by the judging circuit 5, its information is automatically input to
the micromotion device control circuit 11. Then, the nonlinear
crystal is automatically moved by the micromotion device 7 to a
position where the laser beam to be input is received at a
non-damaged region of the nonlinear crystal 1 based on a signal
output from the micromotion device control circuit 11. At this
time, it is more preferable if an operation method is such that
alarm of crystal replacement is automatically generated provided
that the non-damaged region does not already remain on the
nonlinear crystal 1.
(Effects)
[0089] In addition to the effects of the third embodiment, in the
case that it is judged as the movement time of the nonlinear
crystal 1, the crystal is automatically moved; therefore, time loss
is reduced compared to the case where an operator manually moves
and it is possible to increase an operating rate.
[0090] As shown above, the embodiments according to the present
invention are described with reference to specific examples. In the
description of the embodiments, those which are not directly
required for the description of the present invention in the device
configuration, control method, and the like are not explained.
However, necessary device configuration and control method can be
properly selected to be used.
[0091] Otherwise, all operation methods of laser light sources, in
which elements of the present invention are included and designs
are properly changed by those skilled in the arts, are covered in
the scope of the present invention.
* * * * *